The proposed research will examine the interplay between the ovaries, the hypothalamo-hypophyseal-axis, and environmental photoperiod in the control of seasonal breeding in female sheep. The hypothesis that seasonal breeding results from photoperiod-induced changes in response to the negative feedback action of estradiol on tonic LH secretion will be tested. The neural pathways by which photoperiodic cues govern response to estradiol will be examined. In this regard, the role of retinal photoreceptors, the pineal gland, and a retinohypothalamic tract will be explored. Further, we will determine the importance of seasonal changes in sensitivity to the positive feedback effects of estradiol in eliciting the preovulatory LH surge and the seasonal emergence of a photoperiodically controlled, circadian LH surge system. Finally, the feedback regulation of FSH secretion by ovarian steroids will be examined during the estrous cycle and seasonal anestrus. Techniques include radioimmunoassays for serum LH, FSH, estradiol and progesterone; a number of surgical approaches including ovariectomy, section of optic nerves, pinealectomy and deafferentiation (stereotaxic) of specific hypothalamic areas; tracing of retinal projections by autoradiographic localization of tritiated amino acids injected into the eye; and controlled delivery of steroid and protein hormones by means of Silastic implants and intravenous infusion, respectively. It is expected that the results will lead to new insight into control of seasonal breeding, a remarkable process of reversible fertility.